Various types of magnetic field sensing elements are known, including Hall Effect elements and magnetoresistance elements. Magnetic field sensors generally include a magnetic field sensing element and other electronic components. Some magnetic field sensors also include a fixed permanent magnet.
Magnetic field sensors generate an electrical signal representative of a sensed magnetic field. In some embodiments, the magnetic field sensor provides information about a sensed ferromagnetic object by sensing fluctuations of the magnetic field associated with the magnet part of the magnetic field sensor as an object moves within a magnetic field generated by the magnet. In the presence of a moving ferromagnetic object, the magnetic field signal sensed by the magnetic field sensor varies in accordance with a shape or profile of the moving ferromagnetic object.
In other embodiments, the magnetic field sensor has no magnet, and the magnetic field sensor provides information about a sensed object to which a magnet is coupled.
Magnetic field sensors are often used to detect movement of features of a ferromagnetic gear, such as gear teeth and/or gear slots. A magnetic field sensor in this application is commonly referred to as a “gear tooth” sensor.
In some arrangements, the gear is placed upon a target object, for example, a camshaft in an engine, thus, it is the rotation of the target object (e.g., camshaft) that is sensed by detection of the moving features of the gear. Gear tooth sensors are used, for example, in automotive applications to provide information to an engine control processor for ignition timing control, fuel management, and other operations.
In other embodiments, a ring magnet with a plurality of alternating poles, which can be ferromagnetic or otherwise magnetic, is coupled to the target object. In these embodiments, the magnetic field sensor senses rotation of the ring magnet and the target object to which it is coupled.
Information provided by the gear tooth sensor to the engine control processor can include, but is not limited to, an absolute angle of rotation of a target object (e.g., a camshaft) as it rotates, a speed of rotation, and, in some embodiments, a direction of rotation. With this information, the engine control processor can adjust the timing of firing of the ignition system and the timing of fuel injection by the fuel injection system.
Gear tooth sensors can include internal “detectors” that fall into two categories, namely, true power on state (TPOS) detectors, and precision rotation detectors. The two categories are generally distinguished by three characteristics: and ability to distinguish gear teeth from valleys when the gear is not moving, a speed with which they can identify edges of a gear after they are powered up, and the ultimate accuracy of their ability to detect the edges of the gear and place edges of an output signal at the proper times. TPOS sensors are often able to distinguish gear teeth from valleys while precision rotation detectors are not. TPOS detectors are relatively fast but have relatively low accuracy, while precision rotation detectors tend to be slow but have high accuracy.
Precision rotation detectors tend not to provide an accurate output signal (e.g., indication of absolute angle of rotation of an object or speed of rotation) immediately upon movement of the target object from zero rotating speed and/or upon movement slowing to zero rotating speed, but instead provide an accurate output signal only once the target object has moved through a substantial rotation or is moving with substantial speed. For example, in one type of magnetic field sensor described in U.S. Pat. No. 6,525,531, issued Feb. 25, 2003, a positive digital-to-analog converter (PDAC) and a negative digital-to-analog converter (NDAC) track positive and negative peaks of magnetic field signal, respectively, for use in generating a threshold signal. A varying magnetic field signal is compared to the threshold signal. However, the outputs of the PDAC and the NDAC may not be accurate indications of the positive and negative peaks of the magnetic field signal until several cycles of the signal (i.e., signal peaks) occur (i.e., until several gear teeth have passed).
In contrast, a true power on state (TPOS) detector can provide a moderately accurate output signal (e.g., indication of absolute angle of rotation or speed of rotation) shortly after movement of a target object (e.g., camshaft) from zero rotating speed or also shortly before movement slowing to zero rotating speed. Furthermore, even when the target object is not moving, the TPOS detector can provide an indication of whether the TPOS detector is in front of a gear tooth or a valley. The TPOS detector can be used in conjunction with a precision rotation detector, both providing information to the engine control processor.
As described above, the conventional TPOS detector provides an accurate output signal with only a small initial rotation of the target object, and before the precision rotation detector can provide an accurate output signal. The TPOS detector can provide information to the engine control processor that can be more accurate than information provided by the precision rotation detector for time periods at the beginning and at the end of rotation of the target object (e.g., start and stop of the engine and camshaft), but which may be less accurate when the object is rotating at speed. When the object is rotating at speed, the engine control processor can primarily use rotation information provided by the precision rotation detector. In most conventional applications, once the magnetic field sensor switches to use the precision rotation detector, it does not return to use the TPOS detector until the target object stops rotating or nearly stops rotating.
A conventional TPOS detector is described in U.S. Pat. No. 7,362,094, issued Apr. 22, 2008. The conventional TPOS detector includes a comparator for comparing the magnetic field signal to a fixed, often trimmed, threshold signal. The conventional TPOS detector can be used in conjunction with and can detect rotational information about a TPOS cam (like a gear), which is disposed upon a target object, e.g., an engine camshaft, configured to rotate.
An output signal from a conventional TPOS detector has at least two states, and typically a high and a low state. The state of the conventional TPOS output signal is high at some times and low at other times as the target object rotates, in accordance with features on the TPOS cam attached to the target object. Similarly, the output signal from a conventional precision rotation detector has at least two states, and typically a high and a low state.
Gear tooth sensors depend upon a variety of mechanical characteristics in order to provide accuracy. For example, the gear tooth sensor must be placed close to (i.e., at a small air gap relative to) the ferromagnetic gear, teeth and valleys of which it senses as they pass. A larger air gap results in a smaller signal processed by the gear tooth sensors, which can result in noise or jitter in positions of edges of the two-state output signal generated by the gear tooth sensor.
As is known, some integrated circuits have internal built-in self-test (BIST) capabilities. A built-in self-test is a function that can verify all or a portion of the internal functionality of an integrated circuit. Some types of integrated circuits have built-in self-test circuits built directly onto the integrated circuit die. Typically, the built-in self-test is activated by external means, for example, a signal communicated from outside the integrated circuit to dedicated pins or ports on the integrated circuit. For example, an integrated circuit that has a memory portion can include a built-in self-test circuit, which can be activated by a self-test signal communicated from outside the integrated circuit. The built-in self-test circuit can test the memory portion of the integrated circuit in response to the self-test signal and report self-test results when requested.
Some conventional magnetic field sensors, for example, magnetic field sensors used in automotive applications, are limited in the number of electrical connections made to the magnetic field sensors. It is often desirable that magnetic field sensors have as few as two or three electrical connections, wherein two of the electrical connections are for power and ground.
It would be desirable to provide a magnetic field sensor that has as few as two or three electrical connections, that can perform self-tests, and that can report the results of the self-tests while not interrupting a signal representative of a sensed magnetic field (i.e., as the magnetic field sensor operates in normal operation) and while using only the two or three electrical connections.